In the production of crude oil, it is frequently found that the crude oil is sufficiently viscous to require the injection of steam into the petroleum reservoir. Ideally, the petroleum reservoir would be completely homogeneous and the steam would enter all portions of the reservoir evenly. However, it is often found that this does not occur. Instead, steam selectively enters a small portion of the reservoir while effectively bypassing other portions of the reservoir. Eventually, "steam breakthrough" occurs and most of the steam flows directly from an injection well to a production well, bypassing a large part of the petroleum reservoir.
It is possible to overcome this problem with various remedial measures, e.g., by plugging off certain portions of the injection well. For example, see U.S. Pat. Nos. 4,470,462 and 4,501,329, assigned to the assignee of the present invention. However, to institute these remedial measures, it is necessary to determine which portions of the reservoir are selectively receiving the injected steam. This is often a difficult problem.
Various methods have been proposed for determining how injected steam is being distributed in the wellbore. Bookout ("Injection Profiles During Steam Injection," SPE Paper No. 801-43C, May 3, 1967) summarizes some of the known methods for determining steam injection profiles and is incorporated herein by reference for all purposes.
The first and most widely used of the these methods is known as a "spinner survey." A tool containing a freely rotating impeller is placed in the wellbore. As steam passes the impeller, it rotates at a rate which depends on the velocity of the steam. The rotation of the impeller is translated into an electrical signal which is transmitted up the logging cable to the surface where it is recorded on a strip chart or other recording device.
As is well known to those skilled in the art, these spinners are greatly affected by the quality of the steam injected into the well, leading to unreliable results or results which cannot be interpreted in any way.
Radioactive tracer surveys are also used in many situations. With this method methyl iodide (CH.sub.3 I) has been used to trace the vapor phase. Sodium iodide has been used to trace the liquid phase. Radioactive iodine is injected into the steam, and the tracer travels down the well in the steam until it enters the formation. A typical gamma ray survey is run during the tracer injection. Recorded gamma ray intensity curves at any point in the well are then analyzed and the steam velocity is directly calculated.
U.S. Pat. No. 4,223,727 to Sustek discloses a method of estimating injectivity in an injection well by measuring volume of fluid injected with surface metering equipment and radioactive tracers to find injection depth. Both methyl iodine and Krypton 85 are mentioned as being suitable gaseous phase tracers.
U.S. Pat. No. 4,507,552 to Roesner describes a tool for injecting and detecting tracers in an injection well. Use of dual detectors for velocity measurement is mentioned.
A written document entitled "Surveying Steam Injection Wells Using Production Logging Instrument" by Davarzani and Roesner, and carrying on it a date of August 1985 describes the device of U.S. Pat. No. 4,507,552 above. The choice of radioactive tracer is not specified. Applicant believes the authors presented the paper at a geothermal conference in Hawaii in August 1985 and the paper was available in a library in January 1986.
The vapor phase tracers have variously been described as alkyl halides (methyl iodide, methyl bromide, and ethyl bromide) or elemental iodine. Although it has previously been believed that these alkyl halide vapor tracers were not subject to decomposition in the short time periods involved, it has been previously noted that the above materials undergo chemical reactions that dramatically affect the accuracy of the results of the survey in steam injection profiling as described in related application Ser. No. 935,662 (allowance granted but not yet issued).
A method of steam injection profiling with inert gas tracers that teaches away from unstable alkyl halide tracers is described in related application Ser. No. 322,582, which is hereby incorporated by reference, and is assigned to applicant's assignee. Two tracers are required: an inert gas tracer and a liquid soluble tracer. Although use of inert gas tracers eliminates the hydrolysis problem created when methyl iodide is used, inert gas tracers are costly, low intensity, and have long half-lives. In many cases, using two separate tracers creates problems when flow is unstable. Two tracer surveys are required, which increases cost and time, and the results are often not additive.
Historically, high bottomhole temperatures encountered during steam injection prohibit using traditional logging sondes. As a result, steam profiling is 5-10 years behind traditional production logging technology. Consequently, accurate measurement of steam profiles is quite difficult, if not impossible.
There is therefore still a need for an improved, more accurate, less expensive, and simpler method to determine steam vapor and liquid profiles.